I know aluminum has 3 valence electrons and oxygen has 7, but if anyone could explain the chemical formula to me it would be great.The chemical formula of aluminium oxide is Al2O3 An aluminium ion has a 3+ charge, because it loses 3 electrons when it reacts. An oxygen ion has a 2- charge, because it gains 2 electrons when it reacts. In aluminium oxide, the charges of the ions need to cancel out. So... 2 aluminium ions -- 6+ total charge , 3 oxygen ions -- 6- total charge The charges cancel out.You are right about aluminum having 3 valence electrons, but oxygen has 6. Oxygen wants two more to get to 8. So the ions are Al +3 and O-2. The formula is Al2O3Other related question